Ever since 3D printers hit the mainstream, I
dreamed of being able to print high-performance Microwave antenna components. It was still
mostly a dream until I saw the announcement about Rogers Radix™ 3D printable dielectric
UV resin material I got all over-excited and made a video asking if I could have a sample to
try my own printer I was amazed when the lovely folks from Rogers Corporation got in touch
and made me an offer I couldn't possibly refuse! The deal was that if I created a
design for a gradient index antenna lens, they'd get it processed and print some
examples for me. Sounded brilliant! Surely there must be a catch?
"There's a catch" they said, "You have to collect the finished antennas in
person, from a facility in the United States, and we'd like you to make a
video about your experiences" Well AIMEE, that's an offer to
which the only right answer is: "Sure! Brilliant! Let's do it!",
and so the adventure began . This video is sponsored by Rogers Corporation (I received payment, parts
and my travel expenses to the USA) I've used Rogers PCB materials for decades
and they're one of the brands that I trust and rely on. As part of this collaboration I had
the opportunity to get a behind-the-scenes view, follow the manufacturing workflow and interview
some of the hugely impressive team at Rogers and Fortify, their 3D printing partner. I'm not an
antenna designer, nor an RF engineer. I've been experimenting with microwave antennas and systems
for over 50 years, but never professionally. If Rogers can give me the tools and workflow to
turn gradient index antenna lens designs that I've dreamt up in my head into real working components,
then just imagine what a REAL RF design engineer could achieve with Radix! Trust me. If Neil
can produce a successful design this way, then literally ANYONE could.
AIMEE, you're so rude. I asked Karl Sprentall how Rogers is enabling RF
designers to create new and improved solutions for contemporary and future
applications using Radix. Yeah, one of the great things about being a
material supplier is you get to design something that basically expands the trade-offs
that are available to your customer and then watch them come up with
great ways to use your material. A few of the applications that
we're excited about right now: One is low-K (or low dielectric constant)
substrates and so this is you know, think of it as a replacement for foam but because
you're printing it you can print either complex structures into it you can print it to have
good structural rigidity and you can even do things like plated through holes directly into the
substrate which you couldn't do in a normal foam. Gradient Index (GRIN) is obviously
a critical part. One of the most well-known gradient index structures is called
the Luneburg lens. It starts with a dielectric constant of 2 at the center, goes to 1
at the outside. You can put an antenna anywhere around the aperture of it and then steer
it by switching where you're feeding this from. Another use case that we're seeing commonly is
complex geometry 3D parts. This is a structure without a gradient index, but as you can see,
we've got an antenna that you want to have fit directly to a surface with metallization. That's
also possible to do with Radix. The important thing to us is not to have a great low-loss resin
that you then put a low conductivity metal on so we focused on working on technologies that can
give you pure copper on the surface of Radix. My brain was well on the way
to exploding at this point. That would be a rather small explosion. Getting an antenna component with
a position variable refractive index is exciting enough but the possibility of incorporating
3D metallization takes my ideas to another level altogether. The team told me how Rogers
carry out rigorous quality control testing on the resin and how Fortify performs tests
to ensure the printers are maintaining the desired parameters of the cured dielectric
material. I first asked Phil about how the printer technology ensures that the cured
material remains homogeneous and isotropic. Many of the resins that we print are filled with
some type of fiber additive. Particle, fiber, isotropic or not, and those fibers or particles
have a tendency to settle out as you're printing. In order to combat that, Fortify developed
CKM which is a technology that you can think of as plumbing in the system which consistently
circulates, mixes and heats the resin in order to maintain the suspension of those particles
throughout the printing process so that you have the same consistency from the beginning
of the build to the end. OK, the fruit cake pic was a bit lame, but here's a real example of a
print with and without continuous kinetic mixing. I asked Phil about design considerations
when incorporating metalized elements. One thing you should consider when you're
trying to metalize something especially if it's a gradient refractive index
device, is making sure there's a solid surface on which you can metalize. That means leveraging
the 3D printing technology that we have and integrating all types of different geometries
that don't necessarily have to be lattice based so that means you can have a lattice side by side
with a solid such as a skin or a mating feature or a solid component to help you mount it to the
ultimate location where it's going to be in its application. So, if a customer asks you to create
a skin or metallizable surface on their part, your workflow could incorporate that for
them. Yeah, exactly correct, that's what we're doing here. So if you look at this device,
this is a Luneburg lens. On the top side, you can see the exposed gradient refractive
index lattice geometry, but on the back side here we have a solid skin, and this is all done
in the toolset that Fortify has developed and it makes it easy for the RF engineer to
conceptualize their devices and quickly go from concept to 3D printed part without
having to think too much about the process. So now the ball was in my court.
I wanted a compact feed for a parabolic dish to carry out moon
bounce experiments at 10 GHz in x-band. The dish has a focal length to diameter ratio of 0.5 which means an illumination
angle around 112 degrees with an edge taper of -12 dB. I wanted extremely low side lobes so
that the receiver wouldn't see any hot ground or astronomical sources which were away from
the main beam axis. I also wanted the feed to work with a simple linear excitation in a round
waveguide using either a coaxial probe or an oval iris to match from waveguide to a machined
round horn while avoiding the need to generate a hybrid mode in the machined metal part of the
feed. I travelled to Rogers labs next and met Chris who showed me some of the test fixtures and
instruments they used to validate the performance of dielectric materials. Over here is showing some
of our additional capabilities in our R&D lab. What you're looking at right here is a split post
dielectric resonator. What this allows us to do is extract the permittivity of various materials with
dielectric constants anywhere from 2 to say 20, 50 or even 100. Whoa! It's time for a quick
sidebar moment here! A split post dielectric resonator uses two ceramic pucks supported a
fixed distance apart inside a microwave cavity. RF energy is injected using a small loop fed with a
coax line a second identical loop samples a field within the cavity. If you connect the fixture to
a vector network analyzer and set the position of the loops correctly, you can measure the amplitude
and phase response of the cavity and pucks over a range of frequencies. Now if you slide a sample of
dielectric into the air gap, so long as you know its thickness and the size of the gap, you can
de-embed the fixture parameters and extract the characteristics of the sample, as Chris explains.
This is measuring at 10 GHz or X-band. The sample will be inserted into our test fixture. Using some
of our internal software we can then measure the coupon and using our Keysight Agilent Technologies
network analyzers it will extract the s-parameters and an algorithm will then extract the
permittivity of the material and the loss tangent. In addition to this fixture at 10 GHz, we also
have additional fixtures. This one is at 2 GHz, and you'll see it allows a much thicker sample. This one's for 15 GHz. I asked Chris about calibration. He
explained that Rogers has an internal calibration lab team dedicated to ensuring all
the instruments are maintained to the relevant standards. Pretty cool huh? I asked Colby
to explain some of the technical quality assurance tests carried out on production
prints using Radix. The first one of those is from the ASTM D2520 small sample
perturbation test. That involves this waveguide fixture. This waveguide has two irises
brazed in about 2.4 inches apart and that sets up a resonance a little bit under 10 GHz. The
idea is that that resonance will go down with perturbation. Colby explained that as the volume
of the solid sample is known, the reduction in center frequency of the resonance, plus the Q
factor change, can be used to characterize the relative permittivity and loss tangent of the
material. After characterizing the empty cavity and taking down the center frequency of the
resonance and the 3dB points, high and low, the first thing we'll do is take this Rexolite
standard rod. It's a circular rod about 0.060 inches in diameter and we'll insert that into the
cavity and take a measurement there and calculate the Dk and Df of the Rexolite. So that's like an
initial calibration check just to verify that the system is working as it should be? Yeah, exactly
that, and you mentioned the word "calibration", it's a good time to point out that because all
we're looking at here is a resonance, that you don't need to do any calibration of the network
analyzer cables or of the waveguide structures itself it's just a turn on and go. So here's an
example of a solid that we can use to measure the solid characteristics of the dielectric and
that's about 50 mil by 50 mil cross section and we can drop that into the fixture in this way
but also you'll notice this sample is very tall and so we can continue to drop the sample deeper
and deeper into the fixture and look for changes over Z height. So this was printed in this
orientation on the Fortify printer and so we can check over a four inch Z height whether
there's been any settling of the fillers in the material because that would change the Dk over the
height of this toothpick. So oftentimes if we're doing a print that's at least four inches we'll
print some of these what we call "toothpicks" as well and do some of that characterization.
So, you can do that as part of a normal print as a quality control check. Yeah exactly.
Colby then explained how you can create test blocks of different lattice density on the
build plate along with your parts to verify the permittivity of lattices as printed. The
blocks are printed to be a good fit into the chosen waveguide. This is WR90 for tests at
10 GHz. The fixture is connected to a vector network analyzer (VNA) and calibrated.
First with a through connection, then a short, then a delay line of between 30 and 160 degrees
at the chosen frequency using shims. The sample blocks are inserted in turn into a set of
shims and placed into the test fixture. The VNA then measures the response, and the
results are used to calculate the actual permittivity, measuring the loss tangent (Df) is
tough, as the lattices are already full of air and the resin has very low loss. From the phase
change we can calculate the effective Dk of the material based on how much we're slowing the
incident wave down inside the waveguide fixture and the ratio of dielectric to air can
come out of that and from that you get your effective Dk. Can you extract
the loss tangent from the s12, or is it too small? We can certainly try. Right now, the reliable method is to measure
the effective Dk and run this test with a solid standard and compare the effective Dk to the
solid Dk and apply that ratio to the solid Df to determine about where we think our effective
Df is living. We're also doing some work to validate this fixture and to do that
we're going to these thin slabs. We're using both an SPDR method, which is a
split post dielectric resonator and then there's also something called
a Fabry-Perot Open Resonator. A Fabry-Perot Open Resonator uses a pair of spherical RF mirrors,
one fixed, one movable, with an injection port and a sampling port,
both loosely coupled so as not to degrade the unloaded Q factor of the cavity, which can be
over a hundred thousand. Inserting a layer of dielectric material at the central zone affects
the Q factor and resonant frequency. The changed values of those parameters can then be used to
calculate the relative permittivity and the loss tangent of the dielectric sample. At 47 GHz, with
180 mm radius mirrors spaced about 300 mm apart, the frequency varies by 150 kHz per micrometre of
offset. That's less than the half power bandwidth of the unloaded cavity. The equations for
relative permittivity and loss tangent are scary looking. Transcendental
even, but hey, "Computers", right? Now I want to make a Fabry-Perot
resonator of my own AIMEE! So many shiny projects. So few completed.
Harsh. but true. So those are the methods we're using for some
validation that our lattice measurement in-house is accurate. So, then a customer could print test
rods, blocks or coupons alongside the parts on the build plate to validate each of the different
relative permittivity zones? Yeah, exactly that. If you were doing a lens with five different
effective Dks, there is likely room on the build plate to print five of these little swatches
that we could test at X-band and like I mentioned earlier, we also have the capability to test
at S-band, although for low loss materials, Dk should really not change in a measurable way
between S and X-band. Maybe half of one percent at the very most and Df should be pretty predictable
with a curve fit. I was curious about the relation between unit cell size and effective Dk. Fortify
have shared some initial draft results with me, showing the safe area for a range of cell sizes
and cutoffs. This is very much initial results so please don't quote me! Colby then told me how
testing and validation of finished antennas was being done using microwave and mmWave anechoic
chambers and about mechanical and environmental testing of the printed resins. I'm going to be
testing my completed antenna lenses in a field deployment. It's going to be exciting to find
out how they behave in a real-world application, as long as I can keep the spiders out!
While I was working on the design, I split the gradient index into subsections
of constant Dk and I asked Colby what sort of feature size versus subsection size work best
he gave me a slightly quizzical look. That kind of depends on the designer and if this is someone
who is comfortable with defining a material Dk in their simulation as an equation-based calculation
that's based on spatial area in their design, then that would feed directly into a pure gradient with
no steps which is entirely feasible. Wait! What? I made a rookie error, assuming I
need to do a pile of work creating constant Dk zones in the EM solver
and CAD models! Oh dear did anybody tell you that you couldn't do that?
No, but I didn't ask the question, so it serves me right, doesn't
it really? Never assume, or you'll end up like Neil. AIMEE you're 100
percent right as usual. Lesson learned. When you're working with innovative new
processes and materials, it's vital not to make assumptions about any limitations. Simply
providing the equations to define the gradients frees RF engineers and designers from a lot of
unnecessary grunt work. Don't be like me folks, I should have thought of that and
not limited myself. Entirely my bad. Phil gave me a tour of the Fortify Print Lab.
I asked him to take us through the process of setting up one of the printers to produce a
run of my lens designs. This is the reservoir that goes into the machine to hold the resin
for the 3D printing and what we're doing right now is assembling the reservoir so that I can
fill it with resin and put it into the machine, assemble the filter, and we're good to go into
the machine. This is a fully assembled reservoir. I'll bring this over to the 3D printer now. I'm
going to put the reservoir into the machine, seat it, then we close the reservoir manifold, the wiper slides right in. The wiper
will pass across the film during the printing cycle to help to maintain the resin's
particulate from sedimenting out and obscuring the UV light At this point we need to add the
Rogers' material. So, it's been bottle-rolling for a couple hours now, to make sure it's nice
and homogeneously mixed. We'll add just a bit. That's good for now. And we'll get that back on the bottle roller
for the next time we need to add material. At this point we're going to warm up the printer
with this button here and what this does is it starts the circulation and mixing that occurs in
the CKM, which is the continuous kinetic mixing system. We are heating up the agitator and once
that step is complete, which we just saw the check mark there, we're going to fill the reservoir. So
now what happens is we have a quantity of material and a reservoir back here behind the Z-axis and
it distributes material into the reservoir until it reaches a set level in the reservoir and we
maintain that level throughout the build. And so on the left side here you can see the inlet. This
is the inlet fluid flow nozzle on the left side of the manifold. The resin is flowing in through a
mesh filter. The resin flows in from the left to the right and then up the back side here There's
a peristaltic pump that pumps the resin out of the outlet side of the reservoir back into our
circulation system to get reheated and remixed. So, throughout the process, the resin will always
be flowing through the inlet to the outlet. The wiper will be moving periodically throughout
the build as well to help to maintain the homogeneity of the material and help to keep the
film clear of any obstruction so that the UV light will penetrate through and make a good part. All
right so we have our build plate, we're going to assemble it into the system, place it in here and
close the clamp and we see on the user interface that the build plate check box just went from
yellow to green, so we are ready to start a build. Looks like the build is done. Now I'm going to take the build plate off. Phil showed me how the parts are removed
from the build plate with a razor blade, then washed in a solvent to remove any
residual resin. For these small parts of mine, a simple agitator table with
a two-stage wash was perfect. This is a cleaner wash, to get the
last little bit of the resin out. Phil allowed the parts to drain, then
used compressed air to drive out any solvent traces and prepare
the parts of final curing. Well, I think it's fair to say that "That'll
do!" It looks remarkably like the CAD model that I designed. All right, now that we have a clean
part, the first step is to cure it in the UV oven. That uses 405 nm long-wave ultraviolet light
to ensure the resin is completely cured The parts are then finished off
with a bake in a thermal oven. Now that looks absolutely gorgeous!
Just before I flew to the United States to visit Rogers and Fortify, I designed
another gradient index dielectric lens, it's a Mikaelian cylindrical lens, with an index
that varies across the diameter according to an inverse hyperbolic cosine expression. The idea was
to use it as a basis for discussing the production workflow. I submitted the design, and the team
suggested some manufacturability improvements, but then I thought no more about it. Imagine my
amazement when Phil showed me a freshly completed print of three of these lenses. They were just
out of the printer and not washed or cured, but they looked BRILLIANT! A few weeks later
a delivery van arrived with a parcel for me No YouTube video is truly complete without an
unboxing scene. Ooh, this is like birthdays and Christmas all rolled into one. I need to
check the weight so I can get the balance of the mount correct. I'd estimated it in my
CAD program at between 250 and 500 grams, perhaps three-fourths of a pound Wow! These things
feel like they're made from fired ceramic. Amazing That's a little under 13 ounces,
perfect. The texture and finish are just breathtaking! I love how the gyroid
lattice structure looks up close it's gorgeous I think it's fair to say that "That'll do!" One of the applications for the Mikaelian
lens is a handheld source of microwave signals rather like a flashlight. I
built a transceiver into a flashlight body. This was the first one I bought, but
it's a bit on the small side. As The Great Australian philosopher poet (M Dundee)
once said: "Call that a flashlight?" "THIS is a flashlight!!!" Karlo is one of the co-founders of Fortify.
I asked him about their vision and the work with Rogers on Radix. We exist as a company
to bring to market what we consider the first production-capable Additive Manufacturing System.
By combining advanced materials, fibers, fillers, and the unique processing conditions that we
built into our system, we're able to create shapes and tackle applications that no other
Additive Manufacturing company has been able to tackle. Through the creation of this platform,
we've had the fortune of partnering with Rogers who have developed some really interesting
high performance, but very tough-to-print resins like the Dk 2.8 material. By combining
their unique materials and our platform, we're able to produce these shapes, these devices, that
are unmatched in the Additive Manufacturing space. Many of the RF engineers and designers I know,
need to be personally convinced before asking their businesses to invest in the capital
equipment to support a new manufacturing technology. As Karlo put it, they have a "Prove
it to me!" mindset, and quite rightly so. The combination of Rogers' excellent Radix material
and Fortify's advanced printing systems will give engineers and designers an entry point into
the benefits of advanced 3D printed dielectric materials for lenses, foam-replacement substrates
and complex metalized forms, without incurring a huge financial risk. My experience of using the
workflow was immensely positive. One of the best things was being accepted as a peer despite me
being the least intelligent person in the room. It's hard to imagine ANY gathering where
you're NOT the least intelligent person. Unlike AIMEE, the teams at Rogers and Fortify are
flexible, smart, helpful and have a razor-sharp focus on customer success. Full information on how
to get details on Radix and the printing system are on the end screen in the description and on a
card up at the top of the screen. The next step is to show the test results and how I machined the
mounts and cavities for the lenses. That'll be in Part Two, which will appear up THERE. Huge thanks
to Rogers and Fortify, and especially to my host Vitali, whose hands starred in several scenes.
Click the link to find out more about Radix
